One serious injury in Friday morning Humboldt Park explosion

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Two employees mixing chemicals in a biodiesel manufacturing plant in Chicago's Humboldt Park on Friday morning set off an explosion that sent 21 people to hospitals. Two of the injured were in serious condition upon arrival, but their conditions stabilized by midday.

The 9 a.m. explosion caused a partial collapse of the one-story Columbus Foods plant near Grand Avenue and Troy Street and shook nearby buildings, according to witnesses. A plume of smoke smelling strongly of chemicals and sulfur lingered in the air for several minutes afterward.

The explosion was set off in a mixing tank when a worker improperly mixed glycerine and sulfuric acid. A company spokesman said eight employees were inside at the time, but none were seriously hurt. And while the explosion caused extensive damage, officials saw no evidence of fire.

At least five ambulances flocked to the scene for the EMS Plan 1 and Hazmat alarm. Fire Department spokesman Quention Curtistold the Trib that a chemical cloud rose from the explosion, and area buildings had to be evacuated.

Two blocks from the explosion, 19 people who were working in a Metra rail yard complained of strong fumes and irritation to their skin. They were transported to area hospitals after fire officials determined they'd come into contact with chemicals from the explosion that had drifted into the yard. Officials said the chemical is an irritant, but can be washed off.

Across the street from the plant, the dean of students at Uhlich Academy, Cherilyn Thomas, said the lid of the tank landed in the parking lot, leaving a six-inch crater, and a heavy pipe hit the school just a few feet away from a window.

Thomas said it's fortunate that no students were injured.

"Parents send their children to school so that they're safe, and I'm just glad that all our students were inside the building," Thomas said.

Columbus Vegetable Oils has been at that location since 1938, and this is its first accident, according to the spokesman.

Of the two Columbus workers were taken to area hospitals, one was in stable condition and the other suffered only minor injuries, fire officials said.

Metra trains in the area are running again after the Milwaukee District North and Milwaukee District West lines were shut down for nearly two hours, according to a service advisory.

Residents and workers in the area said the explosion shook the ground and rattled windows as far as 3/4 of a mile away.

Eamon Madigan, who was several blocks from the scene, told NBC Chicago he felt the explosion, and it shook the three-story building he was in. Jett Weiner, whose wife works at California and Fulton, said she called to say she also felt the explosion shake her building. Nikki Tyner, who was in an apartment in Humboldt Park, said "the building shook like there was an earthquake."